OBJECTIVE: To describe the frequency and associated factors of high-fat and low-fiber diets among adolescents. METHODS: A cross-sectional study was carried out in adolescents aged 10-12 years in Pelotas, southern Brazil, in 2004 and 2005. Dietary patterns in the previous 12 months were evaluated using the Block questionnaire comprising 24 food items scored according to the frequency of consumption of high-fat and low-fiber food. In the crude analysis, the prevalence of high-fat and low-fiber diets were compared according to subgroups of independent variables (sex, skin color, socioeconomic condition, maternal schooling and adolescent's nutritional status). In order to adjust for confounders, multivariable analysis using Poisson's regression was carried out for each outcome. RESULTS: There were 4,452 adolescents included in the study, most of them (83.9%) had low-fiber diets and more than one third (36.6%) had high-fat diets. Socioeconomic condition and maternal schooling were directly associated with consumption of high-fat diets. Adolescents from socioeconomic groups A+B and C had lower prevalence of low-fiber diet. CONCLUSIONS: The prevalence of low-fiber and high-fat diets was high in this population of adolescents. Public policies targeting the determinants of dietary habits are necessary and urgent.